Dissolved oxygen (DO) is an essential requirement to maintain viable biochemical processes required for water treatment and in maintaining healthy aquatic environments.
There exist today a plenitude of aeration devices (aerators) and aeration methods, principally evolved from and designed for applications within the field of wastewater treatment. Many of these aeration devices and methods have been introduced into other fields of application such as aquaculture for supplying the requirements of dissolved oxygen. Aerators in addition to providing a source of oxygen to be transferred into water also induce turbulent mixing. This turbulent mixing action can in certain applications result in adverse complications, along with increased energy consumption, operational and maintenance needs with a subsequent increase in overall cost.
The two principle types of processes employed for aeration are subsurface aeration and surface aeration. Each type has a number of technologies and variants that perform the task of transferring air into water.
Subsurface or pressure aeration employs a blower or compressor to deliver oxygen under pressure to some form of air transferring device located at a specified depth within the water column. Bubbles that are formed under pressure ascend quickly and generate mixing conditions within the water column. Because pressurized bubbles rise to surface quickly it becomes imperative that pressure type aerators are placed at a sufficient depth within the water column in order to provide adequate gas transfer.
Surface or mechanical aeration, involves rigorous surface agitation forming a water spray of small water droplets wherein oxygen is transferred into the water.
Aspirating type mechanical aerators introduce oxygen into the water by drawing atmospheric air through a draft tube or gas conveyance tube via the action of a rotating propeller. The action of the propeller creates a highly turbulent mixing environment that maintains particles to be in suspension. Propeller type aspirating aerators typically are positioned within the water at a depth between 60 to 120 centimeters and placed at an angle between 25 and 30 degrees.
Other aspirating type aerator variants, which are incorporated below for reference, have been introduced recently they are vertically oriented, partially submerged typically to a depth ranging from 20 to 50 centimeters and are equipped with the impeller positioned adjacent to the bottom bubble discharge end of aerator.
U.S. Pat. No. 6,884,353 B2, Jerard B Hoage, discloses an aeration apparatus to produce small bubbles for use in a septic tank and the like, comprising of an orifice plate having small holes and slots within the air transfer tube and above the impeller.
U.S. Pat. No. 7,306,722 B1, Jerard B Hoage, discloses an aeration apparatus to create small bubbles for use in industrial type wastewater that comprises a rotating disc having louvered openings.
U.S. Pat. No. 7,651,075, Samuel S Rho and Jae-Hak Eorn, discloses an aeration apparatus to produce small air bubbles for use in septic tank wastewater and comprises an aeration disk having angle blades and incorporating an arcuate wall having several slots.
The operating principles of the prior art aspirating type aerators referenced above are similar and rely upon a rotating impeller to produce an area of lower pressure or vacuum allowing atmospheric gas to be drawn into contact and mixed with liquid.
Bubble formation is a product of high shear forces from the rotating impeller. As gas is drawn into the liquid the air-liquid mixture is subject to the shearing forces imparted by the rotating impeller where bubbles are formed at the trailing edge. These bubbles will vary in diameter and are subject to bubble coalescence when discharged, wherein bubbles unite to form larger bubbles that will ascend quickly and reduce gas-hold up or residence time. When the ratio of large bubbles, greater than 1 millimeter in diameter, are high the bubbles generate an uplifting effect as well as changing the liquid density wherein fine to micro sized gas bubbles will rise at a faster rate. This creates the need to have increased gas input volume to offset the loss of oxygen discharged to the atmosphere at the liquid gas boundary thereby reducing aeration efficiency.
The position of the impeller (aeration disk—Rho and Eom), of the prior arts, adjacent to bottom end of the aerator produces a large percentage of gas bubbles that are predominantly greater than one millimeter in diameter having a high ascent rate. A high percentage of quickly ascending bubbles will cause small bubbles of predominantly less than 1 mm in diameter to rise at a higher ascent rate thereby reducing their residence time. In addition the proximity of the impeller at the bottom end of aerator generates radial and helical turbulence that is transferred directly into the liquid body. In the event the aerator is placed within a liquid body containing large amounts of solids there is an increased potential of debris entering the impeller.
It therefore becomes apparent that improvements are required with respect to vertical aspirating aerators wherein aeration efficiency, bubble residence time are increased, turbulent energy transfer is reduced and impeller impacts are prevented.